In the prior art, solid-state image sensors, which generally are composed of a CCD array, are employed as a photosensitive member positioned at the image surface of image readers, such as facsimile machines and image scanners. For image readers, high quality imaging even at the periphery of the image field, with small distortion in red, green, and blue colors on the CCD array is required. It is also required that other aberrations be favorably corrected at a magnification of approximately 0.75 so that high spatial frequencies of the image may be resolved by the image sensor.
As a lens system for reading images which satisfies the requirements described above, JP Patent No. 2790919 and Japanese Laid Open Patent Application H09-304696 describe a lens system for reading images, comprising six lens elements. In addition, Japanese Laid Open Patent Application H09-281459 describes a lens system for reading images, especially narrow angle images.
However, much better images are demanded in recent image readers such as color copiers. Therefore, there is a need of a lens system for reading images which more favorably corrects aberrations so as to achieve higher resolution in image readers.
A first object of the present invention is to provide a lens system for reading images that favorable corrects aberrations even at wide angles, is compact, and yet has a low f-number so as to yield a bright image. A second object of the invention is to provide an image reader that employs such a lens system.